Magnetocrystalline and structural phase transitions of compounds MOCl (M = Ti, V, Cr, Fe) in dependence on pressure and temperature.
Final Report Abstract
Layered compounds MOCl (M = Ti, V, Cr, Fe) are paramagnetic at ambient conditions. Orbital order of the unpaired 3d electrons of the M3+ ions is responsible for the low-dimensional magnetic properties. The orthorhombic FeOCl structure type is frustrated for antiferromagnetic (AFM) order. Upon cooling, a spin-Peierls state develops in TiOCl (Ti3+ has a single 3d electron), and AFM order develops in VOCl, CrOCl and FeOCl. The AFM order is the result of strong magnetoelastic coupling, whereby the magnetic frustration is relieved by a monoclinic lattice distortion. Rising pressure at room temperature leads to phase transitions at a critical pressure of pc ≈ 15 GPa for all MOCl. This is a pure structural transition, without a magnetic component. For p > pc, a modulated, that is aperiodic crystal structure allows for more favorable Cl–Cl distances within the Van-der-Waals gaps between the layers MOCl, than is possible in the FeOCl structure type. In the project we have performed high-pressure (HP), lowtemperature (LT) single-crystal x-ray diffraction (SXRD) experiments on MOCl crystals. FeOCl has a phase transition from paramagnetic to monoclinic–AFM at TN = 81 K under atmospheric pressure. We have found a positive pressure coefficient ∆TN/∆p = 2.13 K/GPa, which can be explained by the pressure dependence of the crystal structure. The behavior of CrOCl is similar, but different on details, including a positive pressure coefficient of the magnetic transition temperatures. The structural transition of MOCl at room temperature and pc ≈ 15 GPa takes place within the orthorhombic paramagnetic phase. The same transition takes place at the same pc ≈ 15 GPa within the monoclinic or triclinic, AFM ordered phases at low temperatures. The monoclinic and triclinic lattice distortions are much larger at high pressure than at low temperatures and atmospheric pressure. This property could have implications for the recently manufactured, single-layer materials MOCl for spintronics, as well as for understanding of Vander-Waals heterostructures. TiOCl undergoes phase transitions to a c-axis-monoclinic, incommensurate spin-Peierls state at Tc2 = 91 K and an a-axis-monoclinic, commensurate spin-Peierls state at Tc1 = 67 K. Including literature data, a preliminary (p,T) phase diagram was constructed. Large positive pressure coefficients, ∆Tc1/∆p and ∆Tc2/∆p, are in agreement with the incommensurate transition taking place at 7 GPa at room temperature. Furthermore, we propose an unusual reentrant behavior of the incommensurate spin-Peierls phase, which forms out of the commensurate phase at temperatures below 200 K and pressures above 8 GPa. For p > pc ≈ 15 GPa, the structural modulation and the incommensurate spin-Peierls state coexist. A comprehensive analysis is presently in progress of the (p,T)-dependence of the crystal structures and possible implications for the magnetic interactions in TiOCl.
Publications
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Broadband CrOCl Saturable Absorber with a Spectral Region Extension to 10.6 µm. Advanced Optical Materials, 8(2).
Wang, Mengxia; Zhang, Jian; Wang, Zhengping; Wang, Ci; van Smaalen, Sander; Xiao, Hang; Chen, Xi; Du, Chenlin; Xu, Xinguang & Tao, Xutang
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Magnetoelastic phase transitions of the unconventional spin-Peierls compound TiOCl at high pressures and low temperatures. Posterbeitrag auf der 27. Jahrestagung der Deutsche Gesellschaft für Kristallographie (DGK), 25.-28. März 2019 in Leipzig.
Schaller, A.M.; Bykov, M.; Bykova, E.; Glazyrin, K. & van Smaalen, S.
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The role of magnetic order in VOCl. Journal of Physics: Condensed Matter, 31(32), 325502.
Ekholm, M.; Schönleber, A. & van, Smaalen S.
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Phase transitions and structural development of FeOCl at high-pressure low-temperature conditions. Posterbeitrag auf der Joint Polish-German Crystallographic Meeting, 24.-27. Februar 2020 in Breslau.
Schaller, A.M.; Bykov, M.; Bykova, E.; Glazyrin, K. & van Smaalen, S.
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High-pressure low-temperature phase transitions and structural development in quasi-two-dimensional transition metal oxychlorides. Acta Crystallographica Section A Foundations and Advances, 77(a2), C537-C537.
Schaller, Achim Mathias; Bykov, Maxim; Bykova, Elena; Glazyrin, Konstantin & van, Smaalen Sander
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Vortrag auf der 29. Jahrestagung der DGK, 15.-18. März, 2021 in Hamburg (Virtuelle Tagung). The lowdimensional magnetic van der Waals compound FeOCl at extreme conditions: pressure and temperature induced phase transitions and structural evolution. e-ISBN (PDF) 978-3-11-074017-2
A. M. Schaller, M. Bykov, E. Bykova, K. Glazyrin & S. van Smaalen
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Pressurizing the van der Waals magnet FeOCl at low temperatures: Phase transitions and structural evolution. Physical Review B, 105(18).
Schaller, Achim M.; Bykov, Maxim; Bykova, Elena; Glazyrin, Konstantin & van, Smaalen Sander
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Pressure-dependent distortions in quasi-two-dimensional magnetic CrOCl at low temperatures. Physical Review B, 108(10).
Schaller, Achim M.; Bykov, Maxim; Bykova, Elena; Glazyrin, Konstantin; Rekis, Toms & van, Smaalen Sander
